(a) Field of the Invention
The present invention relates to an olefin-based polymer having excellent melt strength, and a film including the same.
(b) Description of the Related Art
In general, a polymer film refers to a non-fibrous plate-shaped plastic molded article having a thickness of 0.25 mm ( 1/100 inch) or less. Since polymers are light, have a good barrier property and excellent transparency, and are relatively inexpensive, they are used in almost all fields such as packaging materials, household goods, automobiles, electronic devices, aircraft, etc. Further, polymers are easy to process, and therefore, polymer films are easily manufactured therefrom. In Korea and abroad, synthetic polymers such as polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, etc. have been developed and widely used in polymer films. Currently, many synthetic polymers are used alone or blended as film materials.
Particularly, polyethylene (PE) is divided into low-density polyethylene, high-density polyethylene, and linear low-density polyethylene according to its density and copolymerization, and kinds of branches, and recently, a variety of polyethylene products are also emerging in the commercialized metallocene catalyst system.
Low-density polyethylene was successfully synthesized in 1933 by ICI, and it has been used as an insulation material for military radar due to its noticeably excellent electrical properties. Low-density polyethylene is one of the general-purpose resins, of which application is expanded mainly from various packaging materials. Its major applications include general packaging, agriculture, shrink films, paper coating, etc. Especially, low-density polyethylene is suitable for coating applications because of its excellent melt strength due to long-chain branches.
Meanwhile, linear low-density polyethylene (LLDPE) is prepared by copolymerization of ethylene with alpha-olefin at a low pressure using a polymerization catalyst, and it is a resin having a narrow molecular weight distribution and short chain branches with a constant length, along with the lack of long chain branches. Linear low-density polyethylene films have high strength at break and elongation, and excellent tear strength and falling weight impact strength, in addition to general polyethylene characteristics, and therefore, their use is growing in the fields of stretch films, overwrap films, etc., to which the existing low-density polyethylene or high-density polyethylene has been hardly applied.
Meanwhile, a preparation process of linear low-density polyethylene using 1-butene or 1-hexene as a comonomer is generally performed in a single gas phase reactor or a single loop slurry reactor, and the process shows higher productivity than a process using 1-octene comonomers. However, due to limitations of catalyst and process technologies, the product has physical properties inferior to those of a product obtained by using 1-octene comonomers, and has a narrow molecular weight distribution to show poor processability. To address these process problems, an expensive fluorine-based processing aid is used, but it takes long time to stabilize, and loss of raw materials is significant, which is not economical.
Many efforts have been made to improve these problems, and for example, Korean Patent No. 218,046, Korean Patent No. 223,105, U.S. Pat. Nos. 5,798,424, 6,114,276, and Japanese Patent No. 2,999,162 report that a new magnesium-supported non-metallocene-based catalyst for olefin polymerization has a superior ability to control a molecular structure during olefin copolymerization, such as a comonomer distribution in polymer chains, a molecular weight distribution, etc., and thus it is able to synthesize a so-called “high-strength linear low-density polyethylene”. They report that the high-strength linear low-density polyethylene has falling weight impact strength twice higher than that of films manufactured from general-purpose linear low-density polyethylene which is polymerized in the presence of a Ziegler-Natta catalyst, thereby showing high impact resistance characteristic. However, due to difficulties in the control of a molecular weight distribution in single reactor polymerization, high-strength linear low-density polyethylene has a typical narrow molecular weight distribution, causing a problem of poor processability. To improve this problem, there is an inconvenience that a mixture of the high-strength linear low-density polyethylene with a low-density polyethylene must be used in some cases.
Under this background, there is a continuous demand for a superior product having a balance between physical properties and processability, and improvement thereof is further required.